Charged particle accelerator



Jan. 30, 1962 w. H. BENNETT 3,019,394

CHARGED PARTICLE ACCELERATOR Original Filed Nov. 21, 1955 3 Sheets-Sheet 1 O I \L a D E 28w 4 :0: 1 DLLD: a 3 H J L fi 000 L 5 f 1 I o E I N I I I m Q INVENTOR WILLARD H- BENNETT ATTORNEY Jan. 30, 1962 w. H. BENNETT 3,019,394

CHARGED PARTICLE ACCELERATOR Original Filed Nov. 21; 1955 5 Sheets-Sheet 2 O I ME P LI J h 3A 3; 000

5 f I O 0: (\1

INVENTOR' & WILLARD H. BENNETT ATTORNEY Jan. 30, 1962 w. H. BENNETT 3,019,394

CHARGED PARTICLE ACCELERATOR Original Filed Nov. 21, 1955 3 Sheets-Sheet 3 INVENTOR WILLARD H. BENNETT BYMJW ATTORNEY United States Patent Ofi ice 3,019,394 Patented Jan. 30, 1962 3,019,394 CHARGED PARTICLE ACCELERATOR Willard H. Bennett, 174 Chesapeake St. SW., Washington, DC.

Original application Nov. 21, 1955, Ser. No. 548,283, now Patent No. 2,942,106, dated June 21, 1960. Divided and this application Apr. 27, 1960, Ser. No. 25,176

7 Claims. (Cl. 328235) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention is a division of application Serial No. 548,283, filed November 21, 1955, now Patent No. 2,942,106, and relates to a device for accelerating high energy particles and more particularly to a device for producing intense ion streams within a tube.

The present invention may be used for producing sustained streams of high-velocity ions mixed with electrons comparatively at rest and to accelerate the ions around in a magnetic field whose intensity is azimuthally modulated so that the ions follow paths which approximate a circle. Superimposed on the steady magnetic field is a radio-frequency sinusoidal magnetic field which induces an electromotive force on the cycling ions. These forces can produce a net gain of energy of ions, if at the frequency of the radio-frequency field used, the time of flight of the ions in an orbit is just the period of the radio frequency field. The average magnetic field is made to vary slightly with radius, and the radio-frequency field is frequency-modulated, which can be in a sawtooth manner, to move ions initially in low-energy orbits nearer the axis toward the high-energy orbits lying far from the axis and leaving them circulating there while additional ions are picked up and moved out by succeeding modulation cycles. The ions are accelerated to be useful in producing nuclear reactions.

. It is accordingly an object of the present invention to produce intense directed ion streams.

Another object of this invention is to produce intense ion streams in a highly concentrated form.

A further object of this invention is to produce intense ion streams of the magnetic self-focusing type.

A still further object of this invention is to produce essentially direct currents of ions.

Still another object of this invention is to produce intense ion streams utilizing sinusoidal alternating forces.

A final object of the present invention is to produce ion streams consisting of one or more species of particles.

Other objects and advantages of the invention will hereinafter become more fully apparent from the following description of the annexed drawings, which illustrates the preferred embodiments, and wherein:

FIG. 1 illustrates a diagrammatic view of the components of the device of the invention,

FIG. 2 illustrates a modification of the device illustrated FIG. 1,

FIG. 3 is a diagrammatic sectional view of the device of FIG. 1 taken along line 1-1 of FIG. 1 showing wire loops on opposite sides of an evacuated chamber within a magnetic field, an ion gun, and means for collecting the ions,

FIG. 4 is a modification of the invention showing permanent magnetic pole pieces on opposite sides of an evacuated chamber, and

FIG. 5 is a diagrammatic view illustrating an ion gun which fires particles into the evacuated chamber.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 which illustrates a typical embodiment of an accelerator 10 comprising a disk-shaped evacuated chamber 11 which may be coated on the inside with a conducting film, and positioned in coaxial relationship with the chamber are adjacent D.-C. coils 12 or other means for producing a steady magnetic field across the evacuated chamber. The means for evacuating the chamber is not shown for simplification of the drawing. Located on each side of the evacuated chamber are four wire loops 13, 14, 15 and 16 on opposite sides of the chamber. The wires are shaped to resemble the outer form of a four bladed propeller, but the form of a propeller with any number of blades may be used. The wire loops are placed directly opposite each other on opposite sides of the chamber so that more intense magnetic fields will be set up within the loops and less intense magnetic fields set up in the area between successive loops. The two propeller shaped wires have common connections to a line carrying frequency-modulated radio-frequency currents so that the current through the wire loops will set up an azimuthally periodic radio-frequency field and accelerate the ions between the wire loops. The ions are accelerated by the electromagnetic forces set up within the chamber wherein the electromagnetic forces are produced by magnetic flux lines around the wire loops on each side of the chamber.

In the modification shown in FIG. 2 the evacuated chamber 11 and the propeller shaped loops of wire on opposite sides of the chamber are positioned within a conductive box 17 which maybe copper. The box 17- is positioned within the magnetic field between the D.-C. magnetic coils 12 and confines the magnetic flux lines produced by the wire loops to the area within the box. The Wire loops are energized with high frequency current, usually at radio frequency as will later appear. Additionally, the frequency is preferably modulated. The frequency modulation employed recurrently sweeps from a lower limit to an upper limit, the transition from the upper limit back to the lower limit taking place as ra-.

pidly as possible. Radio frequency generators of this type are well known and for this reason is shown in block form 20 in FIG. 1. The present invention may employ a saw-tooth control wave derived from a multivibrator or blocking oscillator having amplitude and frequency control adjustment and setting devices, and applying the control wave to a reactance tube coupled as a frequency control element of an electron tube oscillator. The frequency modulated output signal is preferably fed to the wire loops through a power amplifier and impedance matching circuit.

The radio frequency current is to be saw-tooth frequency-modulated in such a manner that the lower frequency limit of the current is such that the period of the radio frequency current corresponds to the time of fiight of the species of ions to be accelerated while the ions travel from loop to loop near the axis. The frequency of the higher frequency limit of the saw-tooth corresponds to the time of flight of the ion from loop to loop when the ion is traveling in an orbit near the outer periphery of the evacuated chamber. The action on an ion traveling in an orbit produced by the radio-frequency current in the loops is such that the induced force on the ion varies with the radial distance of the ion from the axis and the phase of the radio frequency current in the coil relative to the position of the ion in the orbit is such that the accelerating force occurs while the ion is in a loop and the decelerating force occur while the ion is in the areas between loops. The resultant effect is to increase the speed of the ion.

FIG. 3 is a view along lines 3-3 of FIG. 1 which illustrates the propeller shapes of the wire loops on each side of the chamber. Further the view illustrates means through which the charged particles can be injected into the chamber and removed therefrom.

FIG. 4 is a modification of the device shown in FIG. I and includes a permanent magnet 18 within which the chamber is positioned. The permanent magnet replaces the D.-C. coils and the device operates the same in all other respects as the device shown in FIG, 1.

Ion sources can be formed in many ways, one of which is illustrated in FIG. 3 as an ion gun which can be any type well known in the art. The ion gun 21 is positioned near the outer edge of the evacuated chamber and at an angle so that it will project a beam of negative ions toward the middle of the chamber from the edge and about midway between sectors, the beam is directed to make the ions strike the wall of the chamber at grazing incidence adjacent to a wire loop. Some of the ions will rebound from the wall as positive ions, moving in approximately the correct direction to be picked up and accelerated by the radio-frequency and magnetic fields.

Another ion source is shown in FIG. wherein the end of a palladium tube 22 is bombarded with electrons from a filament 23 nearby, while the palladium tube is filled with hydrogen.

A screen 24 is interposed between the electrodes and the principal portion of the evacuated chamber toact as a shield in preventing the potential on the electrodes from interfering with the operation of the accelerator. If sufficient electron current is passed at sufficient voltage, the palladium tube will be heated and will transmit hydrogen. The hydrogen upon emerging from the tube will be atomic and will be ionized *by the dense electron current. The atomic hydrogen upon emerging from the palladium tube and being ionized by the electron bombardment is accelerated toward the filament but misses it due to the atcion of the ma netic field and is deflected along the path shown by the broken line 26 in FIG. 5. The axial focusing action of the system makes the ions approach a centered orbit and the next saw-tooth cycle of RF. picks up the ions and moves them toward the outer orbit as disclosed in reference to FIG. 1.

Instead of the palladium tube as shown in FIG. 5 an electron injector could be used to inject electrons into the evacuated chamber. The electrons would be injected perpendicular to the outer orbit and would follow a path toward the axis until it is picked up by radio-frequency alternating current and accelerated to the outer orbit.

Another source of ion arises from the presence of electrons everywhere in the tube. These can be accelerated and decelerated by the radio-frequency field, spending part of their time with energies suflicient to ionize the gas in the tube. This method will supply some ions to the accelerating system. The process may be directly initiated by applying the radio-frequency field with the D.-C. magnetic field off in order to fill the chamber with an electrodeless discharge. After the chamber has been filled with ions and electrons, the D.-C. magnetic field is applied to accelerate the higher velocity ions which produce secondary effects in sufficient density to sustain the ionization needed to serve as an ion source.

The circulating beam of charged particles generated by the present invention may be employed as desired. For instance, the particles may be collected by deflecting them from their reentran-t stable path by applying a control potential to an electrode 27 shown in FIGS. 3 and 5. An attracting potential of a polarity determined by the particle charge will direct the beam outwardly from the stable orbit to a target or collector device. This device may therefore receive particles of particular types accelerated by the system from among a variety of different charged particles present therein. The target or collector may, if desired further or alternately receive the particle at a velocity and energy to effect a physical or nuclear reaction with the target material. An electron beam established by the system may be directed on to a metallic collector to generate X-radiation.

The deflecting signal applied to the deflection device is preferably of short duration where continuous utilization of the system is desired. Upon cessation of the control influence, therefore, the stable outer orbit is reestablished and further particles accelerated thereinto may then be deflected for utilization.

The deflection signal, when of a duration approaching or exceeding the outer orbital period, need not be repeated until a succeeding cycle of frequency modulation accelerates more particles into the outer orbit. It is therefore convenient to derive the deflection control signal from a recurrent pulse generator having control devices. The pulse frequency control device may operate under control of the frequency modulator providing the saw-tooth control wave to supply deflection pulses at a desired phase and in a desired frequency relation thereto. Thus the deflection signal generator may be a normally quiescent blocking oscillator triggered by a signal derived from the saw-tooth oscillator through phase control circuitry, and if desired, count-down frequency dividing circuits, to operate once each saw-tooth cycle or at a lower pulse frequency during spaced successive cycles of the sawtooth generator. It is clear, however, that the deflection signal generator need not be so synchronized because high velocity particles are continually being made available in the outer stable orbit.

For other purposes it may be desirable to intercept the beam when desired by a target probe 28 in FIGS. 3 and 5, which may be movably mounted on the tube envelope by a removable bellows type seal.

It is to be noted that in this invention the orbits of those electrons which are liberated in ionizations will be very small and for this reason the electrons are confined to the immediate vicinity of magnetic lines of force passing through their respective points of origin. It is the ions which are the rapidly moving electnical charges and consequently it is the ions which are magnetically self-focusing and in excess. The electrons produced in the ionizations will be driven out of the mid-plane of the chamber until there are just enough left to neutralize the space charge of the average positive ion density at each position in the mid-plane of the chamber.

It should also be noted that this device can be an electron accelerator by applying radio-frequency fields with the appropriately much higher frequency. It is feasible to apply both an R.-F. field to accelerate ions, and, simultaneously, by appropriate means an R.-F. field of another frequency for electrons. The ratio of the frequencies can be obtained approximately by using the formula w=e/m-H., where w is the angular velocity, the charge of the electron, mmass of theion, and H-the guiding average magnetic field.

A typical example for the operation of the device during acceleration of protons is carried forth by the use of an evacuated accelerating chamber comprising a resilient gas pressure of 10" mm. of Hg, and the mean free path of the ions is of the order of 10 cm. If the average magnetic field applied in the chamber is 1080 gauss, the energy of protons in the outer orbit will be 2,000 volts, the velocity will be 6x10 cm./ sec. and the mean-free time of an ion is 1.7 l0- sec. If the saw-tooth repetition frequency is 25,000 per sec., the current in the outer orbit will decrease 22% during each saw-tooth cycle and the maximum current in the outer orbit will be approximately 4.5 times the average current fed into the chamber from the inner orbits. This is only one typical example of accelerating ions and the invention is not limited to acceleration of pro-tons but may be used for other ions and other valves.

It will be obvious to those skilled in the art that the steady magnetic field can be radially increasing toward the axis and the radio-frequency current can be such that the low frequency limit is at the outer orbit and the high frequency limit near the center of the chamber. In this system the ions would be accelerated from the axis toward the outer orbits of the chamber, but in this case the saw-tooth is from higher to lower frequency instead of the reverse as previously described.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. An accelerator for charged particles comprising an evacuated chamber means, means for producing a magnetic field in which the magnetic lines of force are symmetric with the axis of said chamber means, oppositely disposed propeller-shaped loops of Wire on each side of said chamber means, and a frequency modulated radiofrequency current connected to said loops of wire for producing a sinusoidal magnetic field coaxially superimposed on said steady magnetic field.

2. An accelerator as claimed in claim 1, wherein said means for producing a magnetic field across said chamber means is produced by DC. coils on each side of said chamber means.

3. An accelerator as claimed in claim 1, wherein said means for producing a steady magnetic field across said chamber means is produced by a permanent magnet.

4. An accelertor for charged particles comprising oppositely disposed D.-C. coils adapted to produce a magnetic field therebetween, an evacuated chamber positioned between said D.-C. coils within said magnetic field axially symmetric with said D.-C. coils, oppositely disposed propeller shaped loops of wire positioned on opposite sides of said evacuated chamber between said D.-C. coils and said evacuated chamber, and a radio-frequency alternating current connected to said propeller shaped loops of wire, said radio-frequency alternating current forming a magnetic field of azimuthal modulation in said evacuated chamber.

5. An accelerator as claimed in claim 4, wherein said evacuated chamber and said propeller shaped loops on opposite sides thereof are enclosed in a metal container, said metal container lying between said D.-C. coils and said propeller shaped loops of wire.

6. An accelerator for charged particles comprising a permanent magnet with a spacing between the pole pieces thereof to provide an axially symmetric magnetic field, an evacuated chamber positioned in between the magnetic pole pieces symmetrically with the magnetic field, oppositely disposed propeller-shaped loops of wire positioned on opposite sides of said evacuated chamber between said evacuated chamber and said pole pieces, and a radio-frequency alternating current connected to said propeller shaped loops of Wire, said radio-frequency alternating current forming a magnetic field of azimuthal modulation in said evacuated chamber.

7. An accelerator as claimed in claim 6, wherein said evacuated chamber and said propeller-shaped loops on opposite sides thereof are enclosed in a metal container, said metal container lying between said pole pieces and said propeller-shaped loops of wire.

References Cited in the file of this patent UNITED STATES PATENTS 

